Such seats are already known, in particular as disclosed in FR-A-2694172, in which the nose of the seat i.e. its front edge consists of a movable part compared with the cushion, forming the main part of the seat surface. This movable part is mounted pivotally about a transverse axis at the front part of the seam frame. Its top surface is curved, exhibiting a profile in the shape of an arc of a circle, having said axis as a center, and it cooperates with the front edge of the cushion, having a concave underside with the same profile. On swivelling, the seat nose can retract in part under the cushion, which confers a variable depth upon the seat, while providing some continuity between the top surface of said cushion and the curved nose surface in the various positions of the latter.
A drawback of this system lies in the fact that such continuity means that the top surface of the cushion must be nearly tangent to the curved nose surface. As a result, the front edge of the cushion has the shape of a quite tapered peak in section, which is a real problem as regards the stuffing and the keeping in shape of this peak, which must still be compliant enough not to be prejudicial to the user's comfort. In addition, the user may find it difficult to operate the seat nose forwards so as to move it from a retracted position to an extended position.